The present invention relates to a hot-melt adhesive, and, in particular, to a hot melt adhesive capable of forming a secure bond to substrates having low surface energies.
Prior hot-melt adhesives have been found to be satisfactory in most applications. A hot-melt adhesive must possess several properties to be capable of use as a bonding agent. First, a hot-melt adhesive must be capable of maintaining a strong bond at the desired operating temperatures and in hostile environments. The adhesive should be non-tacky at room temperature so that it can be readily applied in the desired application. Lastly, the adhesive must be fluid enough at its application temperature to wet the surface of the substrate to be adhered to. This is essential for the formation of a secure bond to said substrate. The ability to wet the surface of the substrate is determined by the surface tension of the liquid. The adhesive must have a lower critical surface tension or surface energy than the surface energy of the substrate to enable wetting of the substrate. Most prior art adhesives are therefore unable to form a bond with substrates having low surface energies. Prior art methods of bonding to substrates with an inherently low surface energy thus involve either chemically or electrically etching the surface of the substrate to increase its surface energy, and where the low surface energy is due to contaminants sandblasting the substrate surface to achieve the same effect by removal of said low surface energy contaminants.
One of the few prior art hot-melt adhesives capable of bonding to substrates having a low surface energy is made by Du Pont and marketed under the name of "Hytrel". Hytrel is a block copolymer derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butanediol. Unfortunately, Hytrel requires relatively extensive preparation of the substrate surface in order to obtain a strong bond and furthermore, is thermooxidatively and hydrolytically less stable than is necessary in certain applications. Furthermore, Hytrel has poor electrical properties from the standpoint of resistivity, dielectric strength and dissipation factor which prevents its use in many electrical applications (e.g. under high humidity conditions).